This invention relates to use of novel, high quality brominated styrenic polymers as flame retardants in thermoplastic formulations, and to novel and eminently useful thermoplastic polymer compositions with which such novel, high quality brominated styrenic polymers have been blended.
Brominated polystyrenes are well established as flame retardants for use in thermoplastics, e.g. polybutylene terephthalate, polyethylene terephthalate and nylon (a.k.a. polyamides). Recently, interest has been shown for expanding their use to syndiotactic polystyrene and polycyclohexylene dimethylene terephthalate. Generally, brominated polystyrenes are produced by a reaction between polystyrene and a brominating agent (e.g., bromine or bromine chloride) in the presence of a solvent (e.g., dichloroethane) and a Lewis acid catalyst. Within this broad context, the prior art has developed several processes which strive to obtain a low cost but high performing brominated polystyrene. Low cost is self-explanatory. Performance is predicted by a bromine content (60-67 wt % generally being preferred), a solution color (.DELTA.E=20-35) and a chlorine content (the maximum being 1.5 wt %). The process chosen to produce the brominated polystyrene will determine the particular structure of the brominated polystyrene produced and, thus, its qualities.
The bromine content of a brominated polystyrene is typically the sum of (1) the bromine which is substituted onto the aromatic portions of the polymer, (2) the bromine which is substituted onto aliphatic portions of the polymer, e.g., the polymer backbone or which is present due to alkylation of the aromatic portion of the polymer, and (3) any ionic bromine present, e.g., sodium bromide. The alkylation reaction is catalyzed by the Lewis acid catalyst and uses the reaction solvent (usually a 1-3 carbon atom dihaloalkane) as the alkylating agent. The bromine for (1) is referred to herein as aromatic bromide, while the bromine for (2) is referred to as aliphatic bromide. Even though ionic bromine can contribute to the bromine content, its contribution to the total bromine content is small.
The chlorine content is credited to chlorine which, like the bromine, is part of the polymer structure as an aromatic and/or aliphatic chloride. The use of bromine chloride as the brominating agent is the largest contributor to the chlorine content.
As a universal proposition, it is preferred that the brominated polystyrene have a minimized aliphatic bromide and/or aliphatic chloride, i.e., aliphatic halide, content. The presence of aliphatic halide in the polymer is not desirable as such aliphatic halide is not as thermally stable as the aromatic halide content, and, thus, the aliphatic halide can be easily converted to hydrogen halide, e.g., HBr or HCl, under normal end-use processing conditions. Hydrogen halide, in the presence of moisture, can cause severe corroding of metal process equipment. A brominated polystyrene having almost all aromatic bromide (ar-bromine) will have desirable flame retarding characteristics as the bromine will not leave the aromatic moiety at processing temperatures, but rather, will leave at the very high temperatures which are encountered in the vicinity of an approaching flame front.
Outside of whether or not the halide is present as an aromatic or aliphatic halide, it is also desirable to minimize the total chlorine content of the brominated polystyrene as chlorine is not as efficacious or as stable a flame retardant constituent as is bromine.
The desirability of obtaining a high aromatic bromine content along with a low aliphatic halide and total chlorine content is, unfortunately, not matched by the ability of prior art processes to produce same. Even though the art has proffered many processes which are claimed to produce a superior brominated polystyrene, none have actually been shown to deliver on their promise. See U.S. Pat. Nos. 4,200,703; 4,352,909; 4,975,496 and 5,532,322. A review of the Examples in these patents, which are reported to be actual experiments, shows that a high bromine content, say 68 wt % or above, is not obtained, much less that such could be obtained without a concomitant high aliphatic bromine content, say above 6000 ppm, based upon the total weight of the brominated polystyrene.
Further, the prior art brominated polystyrenes do not exhibit high thermal stability. Prior art polymers exhibit a 1% weight loss at temperatures less than 336.degree. C. when submitted to Thermogravimetric Analysis (TGA) and, indeed, most exhibit a 1% weight loss at temperatures around 300.degree. C. A low thermal stability is not desired when the brominated polystyrene is formulated with thermoplastic formulations which will be exposed to high processing temperatures.
Additionally, it has been demonstrated that prior art processes for the manufacture of brominated polystyrene give rise to significant cleavage or cross-linking of the polymer chain. This cleavage results in the produced brominated polystyrene having an M.sub.w, as measured by Gel Permeation Chromatography, which is significantly lower than the calculated theoretical M.sub.w of the brominated polystyrene. The calculation is based upon the bromine content (wt %) of the brominated polystyrene product and the M.sub.w of the polystyrene reactant at reaction initiation. It is advantageous if the theoretical and actual M.sub.w 's of the produced brominated polystyrene are close, given the.+-.margins of error for GPC, since such closeness evidences a paucity of polymer cleavage. The degree of cleavage should be minimized since cleavage results in an increase of aliphatic end groups in the brominated polystyrene, which end groups provide loci for the facile formation of the undesirable hydrolyzable halides discussed above. Conversely, if cross-linking occurs, the molecular weight of the brominated polystyrene is increased, and if not controlled, such cross-linking can result in formation of insoluble residues and/or gelation of the reaction mixture. In addition, viscosity specifications related to end product usage can be disrupted by such undesirable increases in molecular weight.
It would be of considerable advantage if flame retarded polymer blends containing a more thermally stable brominated styrenic polymer, e.g., brominated polystyrene, and having superior electrical properties could be provided. For example, it would be of advantage if polyalkylene terephthalate compositions could be produced that have (A) a UL-94 rating of V-O using both 1/16+L -inch test specimens and 1/32-inch test specimens, (B) superior electrical resistance characteristics as reflected by a higher comparative tracking index, or (C) superior melt stability as determined by capillary rheometry, without material loss of other necessary and desirable physical and performance characteristics. It would be particularly advantageous if polyalkylene terephthalate compositions could be produced that have a combination of any two or, if possible, all three of (A) through (C) as just described.
Comparative tracking index (CTI) is a measure of the resistance of a material to the propagation of arcs (tracks) along its surface under wet conditions. CTI values considerably lower than that of the control formulation signify lower thermal stability and/or the presence of small amounts of volatile species in the additives, which in turn degrade the polyalkylene terephthalate, reducing molecular weight. These low molecular weight polymer chains can then volatilize and carbonize on the surface, resulting in surface tracking. High melt stability as reflected by capillary rheometry data is indicative of superior thermal stability of the overall polymer composition when used under actual service conditions.